1. Field of the Invention
The present invention relates to a multilayer optical recording medium having three or more recording layers and a method for recording information on the multilayer optical recording medium.
2. Description of the Related Art
Optical recording media such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely utilized as information recording media. Furthermore, some types of optical recording media which utilize a blue or blue-violet laser as the irradiation light to store more information than ever receives attention in recent years.
The optical recording media are generally classified into three types: a ROM (Read Only Memory) type, which can neither record nor rewrite data, a RW (Rewritable) type, which can rewrite data, and an R (Recordable) type, which can record data only once.
In the R-type and RW-type optical recording media, a recording layer is irradiated with laser light and a recording mark which has a reflectivity different from that of surrounding spaces is formed, thereby allowing for the recording of data. Generally in the optical recording medium, the recording layer is irradiated with laser light and a photodetector then measures the difference in reflectance between the recording mark and the space section to reproduce data.
Such optical recording media can have an increased storage capacity if a plurality of recording layers is utilized. For example, in R-type or RW-type optical recording media that have a plurality of recording layers, data can be selectively recorded on a target recording layer when a recording laser beam is focused onto that target recording layer. The data can therefore be selectively reproduced from that target recording layer alone when the reproducing laser beam is focused onto that target recording layer. Furthermore, in a multilayer recording medium having three or more recording layers, it is preferable that the extinction coefficient of each recording layer except for the L0 layer, through which laser light does not need to pass because the L0 layer is the layer farthest away from the recording layer, be 0.5 or less for the wavelength of the laser light used with regard to both transmittance and recording sensitivity.
As described above, in a multilayer optical recording medium, the focus of the laser light is adjusted onto the target recording layer when recording and reproducing data. In such cases, the laser light passes through the recording layers lying on the side of the incident surface of the laser light and onto the target recording layer. It is preferable that transmittance should be constant irrespective of the recording conditions of the recording layers lying between the incident surface and the target recording layer.
In actual fact, however, when data is stored on the recording layer on the incident surface side of the target recording layer, the recording mark area formed varies in both reflectivity, as described above, and transmittance. The inventors discovered that since the amount of laser light passing through a recording mark area of varying transmittance was different from the amount of laser light passing through an area other than the recording mark area, the amount of laser light (being the amount of transmittance) reaching the target recording layer varied and hence the optimal recording sensitivity of the target recording layer also varied. This problem is especially noticeable in a multilayer optical recording medium having three or more recording layers.